Source Parameters of Shallow Microtremors Induced by Seasonal Groundwater Recharge in Hyderabad, Southern Peninsular India

Abstract

Source parameters were estimated using multiple empirical Green’s functions (meGf) method for 2 clusters of shallow depth and small-magnitude earthquakes induced by the infiltration due to gravity into the groundwater zone after heavy monsoons during October 1998 and October 2017, in Hyderabad, South India. First, four decades of earthquake catalogue data recorded by HYB seismic station at CSIR-National Geophysical Research Institute (NGRI) was used to characterize the spatiotemporal variation in seismicity. Enhanced seismicity was noticed because of the strong seasonal fluctuations of the groundwater table during the post-monsoon season. Second, focus was on the estimation of source parameters for two cluster of swarm events whose local magnitude (Ml) are in -0.09<Ml<1.52 range. Stress drops (<1Mpa) was observed in correlation with the seismic moment for both earthquakes sequence. The results indicate that the difference in corner frequency between the two swarm sequences is more than a factor of two, which results in a difference of more than an order of magnitude in the stress drop and seismic moment. The spatial migration of micro-earthquakes is most striking feature observed in both series of microearthquakes because of the diffusion of fluids. Groundwater level response in post monsoon have a significant effect in triggering the earthquakes during the observed time period. The spectrograms are estimated to understand the mechanism of sounds generated by the events with shallow hypocentres during 2017 swarm activity. Spectrogram analysis of the events revealed high energy at frequencies ≥20 Hz. In addition, the particle motion was dominated by retrograde elliptical motion consistent with Rayleigh waves at higher frequencies. This study of source parameter characterization of swarm activity may provide additional information about induced seismicity for seasonal fluctuations.

INSA National Report on Seismological Research in India: 2019 – 2022

“…The 2011 Sikkim earthquake had about five aftershocks of magnitude (mb) ≥ 4.5 between 2011 and 2014 for the same region. Parvin et al (2021) analyzed two swarm activities in the Hyderabad region and found small absolute stress drop values (< 1 MPa) and a positive correlation between the static stress drop and the magnitude of the earthquake with seismic moment varying between -0.09 < M L < 1.52. They observed a clear correlation of earthquakes associated with the fractures and faults in the vicinity of water bodies which are more sensitive to variations in hydrostatic pressures caused by vertical flow recharge from rainfall and deeper pore-fluid pressure diffusion.…”